Linear 3.3V power supply anywhere?

I can guess at why you're not finding one....

1) This is near the drop out voltage of typical regulators 2) Switchers may rule this area 3) At 3.3V ..%temp drift maybe a concern. The internal reference in a hot linear reg may have too much annoying drift. 4) Low customer demand 5) low dropout linear reg + power transistors = any current you want

Here's some quirky wisdom... If you can't find it...you're wrong! :)

(No offense meant..) D from BC

Joerg, I guess this ist the domain of controllers and logic, so the manufacturers assume a switcher to be sufficient for all applications. Though there are no sole 3.3V switchers available here either.

Rene

Drop-out doesn't matter. You just feed in enough volts. It's easy to buy

2V linear supplies, even 0.5V duals. But no 3.3V.

They sure do :-(

No problem. Designed many low voltage regulators with ye olde uA723 or semi-discrete. Rock stable. I just don't want to design another one, this project has enough other work and the missus ain't going to be happy if I tell her that instead of taking her out to our favorite Thai place I'll design a power supply.

Looks like it. Nobody is concerned about noise no more...

No more LDOs in this here consulting office. They've given us too much grief, undocumented pathology mode, explosions etc.

Aw, that hurts. It just piles on to John Larkin's accusation of heresy because we are swinging a VME backplane at 3.3V. Plus we chopped up the bus and run SPI over it, but shhhht, don't tell.

--
Regards, Joerg

http://www.analogconsultants.com

If you ever need any of these Digikey has tons of 3.3V single modules. AFAIK from Astec but as you said they are all switchers. Digikey lists some of them as linear but they aren't.

Thing is, yes, we have to supply logic but that sits in a card cage that must remain super duper whisper quiet. Even the bus must be stopped during signal acquisition.

So, I guess I'll roll my own. Again. Dang. Or modify a 5V but their crude SCR crowbars aren't easy to fix at such low voltages.

--
Regards, Joerg

http://www.analogconsultants.com

I heard that! What idiot suggested that you use a VME backplane in the first place?

Do you want me to design a 5v to 3.3v linear regulator for you? So you don't miss the pad thai and twice-cooked duck?

4 discretes should do it, maybe 3.

John

Guilty. We needed a quick prototype, I spotted a fancy Schroff box with all the bling on EBay same day, the client's engineer pokered by bidding one buck above min and, bingo. I promise, in the final version we'll call it something other than VME. But might still use a VME backplane. If it's any comfort the really unorthodox matrix stuff happens on P2 :-)

Thanks, but I've already butchered a PowerOne 5V supply. The only pain is the crowbar. They use a SCR plus zener. It strays from 0.6-1.5V and the trigger current from 3-32mA. And zeners in the 3V range aren't exactly that great either. Maybe I'll just use a string of diodes.

The nice thing about these PowerOne supplies is that you can write a nice mod procedure for production. Two screws and the circuit board comes off. But tape the transistor in place beforehand.

This Thai restaurant is a tiny place that only the locals can find. But quite genuine and always busy. If I order medium it drives the sweat beads onto my scalp although I am used to very spicy food. Then there is two more grades, hot, and Thai-hot. Oh, and the Singha beer...

--
Regards, Joerg

http://www.analogconsultants.com

[snip] [snip]

Ohh come on.... it does matter a bit.. Let's say the drop out voltage is 3Volts.. Regulator dissipation is 2A*3V=6 watts (or more) A monolithic regulator might have some yucky drift too at 6Watts.. That's nearly = to the load dissipation at 6.6Watts.

I'm saying that a large dropout voltage results in a less efficient linear regulator. D from BC

Sure but we live in Caleefohniah where the power can be anywhere between

110V and 130V, or sometimes 0V. So you need dropout just to accommodate that. Dissipation doesn't matter right now and I am not a friend of monolithic regulators for anything above a few hundred mA. The LM317 is pretty much as far as I will go in that domain. I like the uA723 a lot, plus some nice fat pass transistors.

--
Regards, Joerg

http://www.analogconsultants.com

Oh. I thought it might have been me.

There's a lot of cheap VME on ebay.

Singha Gold is wonderful beer, but hard to find. The regular Singha is pretty good, too.

We like Regent Thai on Church Street, right where they filmed "Sister Act." It's run by all women who wear gorgeous silk Thai dresses. Great food and cheap.

John

Yeah, Sister Mary Clarence, a.k.a. Whoopi Goldberg. One of the best movies. Often critics call it just a "comedy" but that doesn't do it justice. There are a lot of civic (and religious) lessons in there. Everyone can do great things, they just have to be "motivated".

Hey, why are places like Digikey out of stock on so much stuff these days? Couldn't even get a decent paired fuse holder there, for crying out loud. Even for blue 24AWG hookup wire there is a whopping supply of one lone spool left. Arrgh.

--
Regards, Joerg

http://www.analogconsultants.com

The latest linear regulator I've played with is the LP2951. Datasheet says a 3.3V version is available. Of course pass transistors will be needed for 2A. For example: Digikey LP2950ACZ-3.3-ND is in stock. D from BC

We ended up using an LDO regulator off the 5V. Board mounted. Not sure if the 2 Amps will be a problem.

Tam

The National LDO series is where I got my first big black eye. I was already prejudiced against LDO but a client really liked it so they said we should use it. The client is king, so I had to. Fired up the prototype, the output voltage came up sluggishly and a "high amperage smell" developed. Hung a scope onto the rail and it was singing the blues, oscillating like crazy. Turned out it didn't like it when the source impedance got too high but we had no choice. Naturally, that was not mentioned in the data sheet. App Engineering told us more or less "You are right, it cannot handle hi-Z, sorry about that". Great. I forgot the exact part number. IIRC its wasn't LP but something like LM2931. I forgot because I vowed never to design one in again. And I never did, threw away the datasheet.

Not that I want to diss one particular vendor here. I've come across more than one hardcore LDO problem. For example a TPS71550 that "didn't like it" when the supply came up too fast. Phsssst ... BANG. Was that mentioned anywhere? Oh no. TI wanted scope plots and what not. I had sent them the rather simple schematic but they were not willing to throw it onto SPICE. Would have been easy but not for the customer since they do not release the innards anymore. So I voted that one off the island as well ;-)

--
Regards, Joerg

http://www.analogconsultants.com

[snip]

What's the difference between an audio amplifier and a linear regulator?

(I'll be working on the punch line...) D from BC

There are no "Regulatorphooles"?

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

I don't think it is as bad as all that. We had two National LP3964 regulators on the board, with no problems. There were 10 uF Tantalums across the input and output, plus a .22 on the output. In the one case, there was even an RF choke in series with the 5 V feed, on the other side of the input cap, of course. They do tell you that you need a bypass cap with an SER of very close to 1 Ohm.

Tam

But tantalums explode. And the capacitor you see on the schematic, right next to the ldo output pin, isn't usually the only capacitor on that node. There could well be another bunch of microfarads of ceramic bypasses all over the board, with a net esr in the milliohms.

And if you use an aluminum cap, and the temperature drops, the esr goes to hell.

Any not-insane regulator should tolerate esr's down to zero.

John

Just how much design is a 3.3V linear regulator? How about a couple of paralleled 317's?

robert

Hi Jörg,

Where is your dicrete solution now? I'm very sure this task will end like: I used a few 2n2222 and a TLC431 and a (don't know the cheapest 2A transistor on earth) ... Hey, do you know Mr. Roman Black's Black-regulator?

May be this is well beside your mind :-)

Marte